Electrolyte composition and method of use thereof

ABSTRACT

Provided herein is an electrolyte composition including a metal silicate or a metal aluminate, a metal phosphate, zinc oxide particles, and a complexing agent useful for plasma electrolytic oxidation treatment of a surface of a metal substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Patent Application No. 63/069,214 filed on Aug. 24, 2020, the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to an electrolyte composition for plasma electrolytic oxidation (PEO) treatment of a surface of a metal substrate and a method of using the electrolyte composition in the PEO treatment of a surface of a metal substrate.

BACKGROUND

Aluminum and its alloys are widely used in the aerospace and transportation industries, because of their desirable properties, such as high strength-to-weight ratio, good formability, and lightweight. However, despite the presence of an aluminum oxide natural passivating film on the surface of aluminum substrates, aluminum and its alloys can still be susceptible to corrosion, especially in the presence of oxidants and caustic agents.

Surface treatment techniques, such as anodic oxidation, PEO, chemical conversion, electrodeposition, and laser-cladding are viable means to mitigate corrosion of aluminum and its alloys. In particular, PEO, a plasma-assisted electrochemical process that tends to produce better corrosion resistance than other techniques (due in part to the thick aluminum oxide films produced) is environmentally friendly and efficient process for surface treatment of aluminum and its alloys. However, the corrosion resistance of PEO coatings is inferior to that of bulk alumina as a result of the porous nature of the oxide coating structure, which allows penetration of corrosive ions and results in incomplete crystallization. As a result, the corrosion resistance of PEO coatings normally only last for seven days as determined by the impression test in 3.5% salt solution. Thus, there is a strong need to develop improved methods for PEO treatment of metal substrates, such as aluminum and its alloys that overcome at least some of the aforementioned challenges.

SUMMARY

Provided herein is an electrolyte composition useful for PEO treatment of a surface of a metal substrate and methods of use thereof The methods described herein can yield substrate PEO coatings with improved density, hardness, and wear and corrosion resistance.

In a first aspect, provided herein is an electrolyte composition comprising a metal silicate or a metal aluminate, a metal phosphate, zinc oxide particles, and a complexing agent.

In certain embodiments, the electrolyte composition further comprises an aqueous solvent.

In certain embodiments, the metal silicate or metal aluminate is sodium silicate, calcium silicate, tricalcium silicate, tricalcium aluminate, tricalcium iron aluminate, potassium silicate, or a mixture thereof.

In certain embodiments, the metal phosphate is selected from the group consisting of sodium hexametaphsophate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium polyphosphate, trisodium phosphate, and sodium pyrophosphate.

In certain embodiments, the zinc oxide particles are micron-sized zinc oxide, nano-sized zinc oxide particles, or a mixture thereof.

In certain embodiments, the complexing agent is ethylene diamine tetraacetic acid (EDTA), triethanolamine, sodium tartrate, citrate, oxalate, or a mixture thereof.

In certain embodiments, the metal phosphate is selected from the group consisting of sodium hexametaphsophate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium polyphosphate, trisodium phosphate, and sodium pyrophosphate; the zinc oxide is micron zinc oxide, nano zinc oxide, or a mixture thereof and the complexing agent is EDTA, triethanolamine, sodium tartrate, citrate, oxalate, or a mixture thereof.

In certain embodiments, the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA.

In certain embodiments, the metal silicate or metal aluminate, the metal phosphate, the zinc oxide particles, and the complexing agent are present in the aqueous solution at a concentration of about 5 to about 50 g/L, about 1 to about 30 g/L, about 2 to about 25 g/L, and about 1 to about 20 g/L, respectively.

In certain embodiments, the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA.

In certain embodiments, the aqueous solvent has a pH of about 6 to about 12.

In certain embodiments, the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; and the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 10 to about 20 g/L, about 10 to about 20 g/L, about 5 to about 15 g/L, and about 10 to about 20 g/L, respectively.

In certain embodiments, the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 15 g/L, about 15 g/L, about 10 g/L, and about 15 g/L, respectively; and the aqueous solvent has a pH of about 6 to about 12.

In a second aspect, provided herein is a method of plasma electrolytic oxidation (PEO) treatment of a surface of a metal substrate, the method comprising: providing an anode comprising the metal substrate; an anode; and an electrolyte solution comprising the electrolyte composition described herein, wherein the electrolyte solution is in contact with the cathode and the anode; and applying an electric current between the cathode and the anode resulting in the PEO of at least a portion of the surface of the metal substrate.

In certain embodiments, the metal substrate is aluminum or an alloy thereof.

In certain embodiments, the step of applying the electric current comprises applying a constant current using at about 5 to about 20 amps/dm² and frequency of about 50 to about 3,000 Hz or a constant voltage of about 300 to about 800 volts at a frequency of about 50 to about 3,000 Hz.

In certain embodiments, the step of applying the electric current comprises applying a constant current using about 20 amps/dm² and frequency of about 100 Hz.

In certain embodiments, the electrolyte solution comprises the metal silicate or metal aluminate, the metal phosphate, the zinc oxide particles, and the complexing agent are present in the aqueous solution at a concentration of about 5 to about 50 g/L, about 1 to about 30 g/L, about 2 to about 25 g/L, and about 1 to about 20 g/L, respectively.

In certain embodiments, the electrolyte solution comprises the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 15 g/L, about 15 g/L, about 10 g/L, and about 15 g/L, respectively; and the aqueous solvent has a pH of about 6 to about 12.

In certain embodiments, the electrolyte solution comprises the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 15 g/L, about 15 g/L, about 10 g/L, and about 15 g/L, respectively; and the aqueous solvent has a pH of about 6 to about 12; and the step of applying the electric current comprises applying a constant current using about 20 amps/dm² and frequency of about 100 Hz.

Other aspects and advantages of the invention will be apparent to those skilled in the art from a review of the ensuing description.

BRIEF DESCRIPTION OF DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of the invention, when taken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts the surface morphology of LY12 aluminum alloy samples after subjecting them to PEO (a) in the absence of zinc oxide; and (b) in the presence of 10 g/L of zin oxide. The size of the aluminum alloy sample was 25×50×2 mm³. The PEO treatment was conducted under 20 KW power supply with a constant current for 15 minutes. The current density was 20 A/dm² and the frequency was 100 Hz. The electrolyte composition comprises sodium silicate (15 g/L), sodium hexametaphosphate (15g/L), Na₂EDTA, and zinc oxide (0-10 g/L).

FIG. 2 depicts the surface morphology of the aluminum alloy samples of FIG. 1 after subjecting them to the corrosion test for 14 days. (a) The sample without zinc oxide doping was eroded after immersion in 3.5% NaCl solution for 14 days. (b) In contrast, the aluminum alloy samples prepared with zinc oxide at a concentration of 10 g/L were not eroded.

FIG. 3 depicts a series of optical images showing the appearance of the surface of untreated LY12 alloy samples and PEO-treated LY12 alloy samples treated with electrolyte composition solutions comprising 0, 2, 4, 6, 7, and 10 g/L of zinc oxide in 3.5 wt % NaCl solution for 0, 2, 7, and 14 days. The results indicate incorporation of zinc oxide significantly increases the corrosion resistance of the PEO coating.

DETAILED DESCRIPTION

Definitions

The definitions of terms used herein are meant to incorporate the present state-of-the-art definitions recognized for each term in the field of biotechnology. Where appropriate, exemplification is provided. The definitions apply to the terms as they are used throughout this specification, unless otherwise limited in specific instances, either individually or as part of a larger group.

Throughout this specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. It is also noted that in this disclosure and particularly in the claims and/or paragraphs, terms such as “comprises”, “comprised”, “comprising” and the like can have the meaning attributed to it in U.S. Patent law; e.g., they can mean “includes”, “included”, “including”, and the like; and that terms such as “consisting essentially of” and “consists essentially of” have the meaning ascribed to them in U.S. Patent law, e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the invention.

Furthermore, throughout the specification and claims, unless the context requires otherwise, the word “include” or variations such as “includes” or “including”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

The present disclosure provides an electrolyte composition comprising a metal silicate or a metal aluminate, a metal phosphate, zinc oxide particles, and a complexing agent.

The electrolyte composition may comprise a metal silicate, a metal aluminate, or a mixture thereof. The metal silicate can be an ortho silicate, a pyro silicate, a cyclic silicate, a chain silicate, or mixture thereof. Metal aluminates suitable for use in the electrolyte composition described herein include, but are not limited to, an aluminate, an aluminate hydrate, and mixtures thereof.

The metal silicate and metal aluminate can comprise any metal cation. Exemplary metal cations include one or more cations selected from Group 1 and Group II of the Periodic Table of the Elements. In certain embodiments, the metal silicate and metal aluminate comprises one or more metal cations selected from the group consisting of Li⁺, Na⁺, Mg²⁺, and Ca²⁺. In certain embodiments, the metal silicate or metal aluminate is selected from the group consisting of sodium silicate, calcium silicate, tricalcium silicate, tricalcium aluminate, tricalcium iron aluminate, potassium silicate, and mixtures thereof. In certain embodiments, the metal silicate is sodium silicate.

Metal phosphates suitable for use in the electrolyte composition described herein include, but are not limited to, phosphate salts, metaphosphate metal salts, triphosphate salts, and polyphosphate metal salts. Exemplary polyphosphate metal salts include, but are not limited to, pyrophosphate salts and polyphosphate salts, such as triphosphate salts, tetraphosphate salts, pentaphosphate salts, trimetaphosphate salts, tetrametaphosphate salts, and the like.

The metal phosphate can comprise any metal cation. Exemplary metal cations include one or more cations selected from Group 1 and Group II of the Periodic Table of the Elements. In certain embodiments, the metal phosphate comprises one or more metal cations selected from the group consisting of Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺. In certain embodiments, the metal phosphate is a sodium hexametaphsophate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium polyphosphate, trisodium phosphate, and sodium pyrophosphate. In certain embodiments, the metal phosphate is sodium hexametaphsophate.

Conjugate acids of the metal phosphate may also be used in the electrolyte compositions described herein. Exemplary conjugate acids of orthophosphate, pyrophosphate, and polyphosphate suitable for use include, but are not limited to, linear polyphosphoric acids, metaphosphoric acids, and branched polyphosphoric acids. Exemplary polyphosphoric acids include, but are not limited to, triphosphoric acids, tetraphosphoric acids, pentaphosphoric acids, trimetaphosphoric acid, tetrametaphosphoric acids, and the like.

The conjugate acids of the metal phosphate can comprise one or more ionizable protons and thus can exist in one or more conjugate acid protonation states. In instances in which the electrolyte composition comprises a conjugate acid of orthophosphate, pyrophosphate, or polyphosphate, the conjugate acid can be in any of the possible protonation states of the phosphate salts described herein or a combination thereof. For example, conjugate acids of PO₄ ³⁻ (orthophosphate) include HPO₄ ²⁻, H₂PO₄ ⁻, and H₃PO₄; and conjugate acids of P₃O₁₀ ⁵⁻ (tripolyphosphate) include HP₃O₁₀ ⁴⁻, H₂P₃O₁₀ ³⁻, H₃P₃O₁₀ ²⁻, H₄P₃O₁₀ ¹⁻, and H₅P₃O₁₀. Anionic conjugate acids of the phosphate salts can comprise any one or more of the metal cations described herein.

Zinc oxide particles suitable for use in the electrolyte composition described herein include nano-zinc oxide, micro-zinc oxide, and mixtures thereof. The nano-zinc oxide can have an average particle size between 50-1000 nm. The micro-zinc oxide can have an average particle size between 1-10 μm.

The complexing agent can be any metal complexing agent known in the art. Exemplary complexing agents include, but are not limited to, nitrilotriacetic acid (NTA), nitrilo(diacetic)propionic acid (NDAP), methylglycindiacetic acid (MGDA), EDDA, 2,2′-(ethylenediimino)-dibutyric acid (EDBA), EDTA, diethylenetriaminepentaacetic acid (DTPA), 1,2-cyclohexane diaminetetraacetic acid (CDTA), ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA), 2,3,4,5,6,7-hexahydroxyheptanoic acid, triethanolamine, tartartic acid, citratic acid, oxalic acid, or conjugate salts thereof and/or mixtures thereof. In certain embodiments, the complexing agent is EDTA, triethanolamine, sodium tartrate, citrate, oxalate, or a mixture thereof. In certain embodiments, the complexing agent is a conjugate salt of EDTA. Conjugate salts of the complexing agent can comprise one or more one or more cations selected from Group 1 and Group II of the Periodic Table of the Elements, such as Li⁺, Na⁺, Mg²⁺, and Ca²⁺.

The electrolyte composition may further comprise an aqueous solvent, such as water. The water can be distilled water or deionized water. The pH of the aqueous solvent can range from about 6 to about 12. The ion conductivity of the electrolyte composition further comprising the aqueous solvent can range from about 5 to about 60 mS·cm⁻¹.

The electrolyte composition can comprise the metal silicate or metal aluminate at a concentration of about 5 to about 50 g/L in the aqueous solvent. In certain embodiments, the electrolyte composition comprises the metal silicate or metal aluminate at a concentration of about 5 to about 40 g/L, about 5 to about 30 g/L, about 5 to about 20 g/L, about 10 to about 20 g/L, or about 15 g/L in the aqueous solvent.

The electrolyte composition can comprise the metal phosphate at a concentration of about 5 to about 50 g/L in the aqueous solvent. In certain embodiments, the electrolyte composition comprises the metal phosphate at a concentration of about 5 to about 40 g/L, about 5 to about 30 g/L, about 5 to about 20 g/L, about 10 to about 20 g/L, or about 15 g/L in the aqueous solvent.

The electrolyte composition can comprise the zinc oxide particles at a concentration of about 1 to about 25 g/L in the aqueous solvent. In certain embodiments, the electrolyte composition comprises the zinc oxide particles at a concentration of about 1 to about 20 g/L, about 5 to about 20 g/L, 5 to about 15 g/L, about 7 to about 13 g/L, or about 10 g/L in the aqueous solvent.

The electrolyte composition can comprise the complexing agent at a concentration of about 5 to about 50 g/L in the aqueous solvent. In certain embodiments, the electrolyte composition comprises the complexing agent at a concentration of about 5 to about 40 g/L, about 5 to about 30 g/L, about 5 to about 20 g/L, about 10 to about 20 g/L, or about 15 g/L in the aqueous solvent.

In certain embodiments, the electrolyte composition comprises the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 15 g/L, about 15 g/L, about 10 g/L, and about 15 g/L, respectively.

Also provided herein is a method of plasma electrolytic oxidation (PEO) treatment of a surface of a metal substrate, the method comprising: providing an anode comprising the metal substrate; an anode; and an electrolyte solution comprising the electrolyte described herein, wherein the electrolyte solution is in contact with the cathode and the anode; and applying an electric current between the cathode and the anode resulting in the PEO of at least a portion of the surface of the metal substrate.

The metal substrate can be aluminum, magnesium, titanium, beryllium, zirconium, and alloys thereof. In certain embodiments, the substrate is aluminum alloy LY12.

In certain embodiments, the step of applying the electric current comprises applying a constant current using at about 5 to about 20 amps/dm² and frequency of about 50 to about 3,000 Hz or a constant voltage of about 300 to about 800 volts at a frequency of about 50 to about 3,000 Hz.

In certain embodiments, the step of applying the electric current comprises applying a constant current using about 20 amps/dm² and frequency of about 100 Hz.

Advantageously, PEO treatment of a metal substrate using the electrolyte composition described herein incorporates zinc oxide into the PEO coating applied to the surface of the substrate, which fuses with the metal substrate to form a denser PEO coating. The result of corrosion tests shown in FIG. 2 demonstrate that the zinc oxide incorporated into PEO coatings resulted in improved corrosion resistance compared to the substrate or PEO coating without zinc oxide. Based on these results, it is clear that zinc oxide incorporation into the PEO coating assists with protecting the coating from corrosion in the NaCl solutions.

EXAMPLES

Commercial polished LY12 Al alloy was used as the substrate for PEO. Before PEO, the substrate was cut into plates (25×50×2 mm) and cleaned ultrasonically in acetone and ethanol for 10 min. The deionized aqueous solution containing sodium silicate (Na₂SiO₃, 15 g/L) and sodium hexametaphosphate ((NaPO₃)₆, 10 g/L) was used as the electrolyte. ZnO particles (300-500 nm, Aladdin Inc., China) with concentrations varying from 0 to 10 g/L were added to the electrolyte and the samples were labeled as ZnO-0 (FIG. 1(a) and FIG. 2(a)), ZnO-2, ZnO-4, ZnO-6, ZnO-8, and ZnO-10 (FIG. 1(b) and FIG. 2(b)), respectively, where the number represented the concentration of zinc oxide in the electrolyte. PEO was performed on a custom system comprising a DC pulsed power supply (Plasma Technology Ltd., Hong Kong) and stainless steel solution container serving as the cathode. The process was conducted at a constant positive current density of 5 A*dm⁻², frequency of 100 Hz, and duty cycle of 30% for 10 min. During PEO, the temperature of the electrolyte was controlled to be below 55° C. with a mechanical stirrer 

What is claimed is:
 1. An electrolyte composition comprising a metal silicate or a metal aluminate, a metal phosphate, zinc oxide particles, and a complexing agent.
 2. The electrolyte composition of claim 1 further comprising an aqueous solvent.
 3. The electrolyte composition of claim 1, wherein the metal silicate or metal aluminate is sodium silicate, calcium silicate, tricalcium silicate, tricalcium aluminate, tricalcium iron aluminate, potassium silicate, or a mixture thereof.
 4. The electrolyte composition of claim 1, wherein the metal phosphate is selected from the group consisting of sodium hexametaphsophate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium polyphosphate, trisodium phosphate, and sodium pyrophosphate.
 5. The electrolyte composition of claim 1, wherein the zinc oxide particles are micron-sized zinc oxide, nano-sized zinc oxide particles, or a mixture thereof.
 6. The electrolyte composition of claim 1, wherein the complexing agent is ethylene diamine tetraacetic acid (EDTA), triethanolamine, sodium tartrate, citrate, oxalate, or a mixture thereof.
 7. The electrolyte composition of claim 1, wherein the metal phosphate is selected from the group consisting of sodium hexametaphsophate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium polyphosphate, trisodium phosphate, and sodium pyrophosphate; the zinc oxide is micron zinc oxide, nano zinc oxide, or a mixture thereof; and the complexing agent is EDTA, triethanolamine, sodium tartrate, citrate, oxalate, or a mixture thereof.
 8. The electrolyte composition of claim 1, wherein the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA.
 9. The electrolyte composition of claim 2, wherein the metal silicate or metal aluminate, the metal phosphate, the zinc oxide particles, and the complexing agent are present in the aqueous solution at a concentration of about 5 to about 50 g/L, about 1 to about 30 g/L, about 2 to about 25 g/L, and about 1 to about 20 g/L, respectively.
 10. The electrolyte composition of claim 9, wherein the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA.
 11. The electrolyte composition of claim 2, wherein the aqueous solvent has a pH of about 6 to about
 12. 12. The electrolyte composition of claim 2, wherein the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; and the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 10 to about 20 g/L, about 10 to about 20 g/L, about 5 to about 15 g/L, and about 10 to about 20 g/L, respectively.
 13. The electrolyte composition of claim 2, wherein the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 15 g/L, about 15 g/L, about 10 g/L, and about 15 g/L, respectively; and the aqueous solvent has a pH of about 6 to about
 12. 14. A method of plasma electrolytic oxidation (PEO) treatment of a surface of a metal substrate, the method comprising: providing an anode comprising the metal substrate; an anode; and an electrolyte solution comprising the electrolyte composition of claim 1, wherein the electrolyte solution is in contact with the cathode and the anode; and applying an electric current between the cathode and the anode resulting in the PEO of at least a portion of the surface of the metal substrate.
 15. The method of claim 14, wherein the metal substrate is aluminum or an alloy thereof.
 16. The method of claim 14, wherein the step of applying the electric current comprises applying a constant current using at about 5 to about 20 amps/dm² and frequency of about 50 to about 3,000 Hz or a constant voltage of about 300 to about 800 volts at a frequency of about 50 to about 3,000 Hz.
 17. The method of claim 14, wherein the step of applying the electric current comprises applying a constant current using about 20 amps/dm² and frequency of about 100 Hz.
 18. The method of claim 14, wherein the electrolyte solution comprises the metal silicate or metal aluminate, the metal phosphate, the zinc oxide particles, and the complexing agent are present in the aqueous solution at a concentration of about 5 to about 50 g/L, about 1 to about 30 g/L, about 2 to about 25 g/L, and about 1 to about 20 g/L, respectively.
 19. The method of claim 17, wherein the electrolyte solution comprises the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 15 g/L, about 15 g/L, about 10 g/L, and about 15 g/L, respectively; and the aqueous solvent has a pH of about 6 to about
 12. 20. The method of claim 15, wherein the electrolyte solution comprises the metal silicate or the metal aluminate is sodium silicate, the metal phosphate is sodium hexametaphosphate, and the complexing agent is disodium EDTA; the sodium silicate, sodium hexametaphosphate, zinc oxide particles, and the disodium EDTA are present in the aqueous solvent at a concentration of about 15 g/L, about 15 g/L, about 10 g/L, and about 15 g/L, respectively; and the aqueous solvent has a pH of about 6 to about 12; and the step of applying the electric current comprises applying a constant current using about 20 amps/dm² and frequency of about 100 Hz. 